The embodiments described herein relate to an X-ray CT apparatus and an X-ray tube current determining method, and particularly to a technique for determining a tube current of an X-ray tube at the time that the heart is imaged or photographed by an X-ray CT (Computed Tomography) apparatus.
Even of X-ray CT apparatuses, one called a VCT (Volume CT), an MDCT (Multi-row Detector CT) or a multi-slice CT or the like is configured so as to detect a cone beam X-ray radiated from an X-ray tube by means of a multi-row detector having 64 rows to 256 rows.
This type of X-ray CT apparatus is used with emphasis on 3D-image photography of the heart or circulatory system by paying attention to the size of coverage in a body-axis direction and the level or degree of efficiency of a helical scan derived therefrom (refer to, for example, Japanese Unexamined Patent Publication No. 2002-330961 (paragraph numbers 0038-0042 and FIG. 14)).
When the range of a helical scan is long in the direction of a body axis as in the case where, for example, a main artery system is imaged or photographed from its main portion to its end, a tube current of an X-ray tube is dynamically adjusted according to the progress of the helical scan in such a manner that the dose of X rays is optimized even at any position on the body axis. The adjustment to the tube current is conducted based on an index of image noise (refer to, for example, Japanese Unexamined Patent Publication No. 2006-116137 (paragraph numbers 0040-0065 and FIG. 5)).
Such a tube current adjustment is also called “automA” or “smartmA”. If the automA and the smartmA are taken, then the image noise becomes constant regardless of the body type or the like of a patient and image quality is stabilized.